The present invention relates to a method for thermally treating articles containing an alloy based on aluminum.
Various methods have been developed in the prior art for improving the resistance to corrosion under certain service conditions of the precipitation hardened condition of aluminum alloy 7075. This condition is referred to as the T6 condition of alloy 7075. However, none of these prior art methods is completely suitable for its intended purpose.
For example, Sprowls et al U.S. Pat. No. 3,198,676 describes a two-step method for improving the resistance of precipitation hardened 7075 alloy to stress-corrosion cracking. Aluminum alloy treated according to the method of the Sprowls et al patent is in a condition that is referred to as the T73 temper. Aluminum 7075 alloy in the T73 temper has improved resistance to stress-corrosion cracking although the T73 temper decreases tensile strength significantly compared with the T6 condition.
Specimens of commercially prepared aluminum 7075 alloy in the T73 temper have sometimes been subjected to a third aging step in order to increase tensile strength and yield strength. For example, a 7075 specimen subjected to precipitation hardening for six hours at 225.degree. F. and heat treatment for eight hours at 350.degree. F. had a tensile strength of 72 ksi (kilopounds per square inch) and a yield strength of 61 ksi. Further precipitation hardening for 48 hours at 250.degree. F. increased tensile strength to 76 ksi and yield strength to 66 ksi. Resistance to stress-corrosion cracking was retained. Although the time of the heat treatment step has been varied between six hours and nine hours in this procedure, applicant is not aware of any prior art three-step aging process wherein the times and temperatures for the second (heat treatment) step correspond to the times and temperatures for the second step of the three-step process described and claimed herein.
Another prior art two-step method for heat treating aluminum alloys is disclosed in Nock et al U.S. Pat. No. 2,248,185. The times and heat treating temperatures described for both of the steps of the Nock et al method are comparable to the times and temperatures employed in the first two steps of the method of the present invention. However, the Nock et al patent does not suggest addition of a third, precipitation hardening step.
Cina U.S. Pat. No. 3,856,584 describes a method that is claimed to reduce the susceptibility to stress-corrosion cracking of 7000 series aluminum alloys. Alloys are successively subjected to a solution heat treatment, an age hardening step at a lower temperature than the heat treatment step, a "retrogression" heat treatment for a few seconds to a few minutes at a temperature of 200.degree. to 260.degree. C. and a reaging heat treatment at a temperature of 115.degree. to 125.degree. C. The thermal aging method of the present invention differs from the method claimed by Cina in that Cina's "retrogression" heat treatment is carried out at higher temperatures than are preferred herein. The present method has the advantages of being easier to control and being suitable for treatment of articles having greater thicknesses than Cina's method.